Electrolytic production of tin from alkali stannate or alkali stannite lyes



Patented May 10, 1932 UNITED STATES. PATENT OFFICE WERNER FANSELA'Q', F BERLIN-SIEMENSSTAQT, GERMANY, ASSIGNOR TO SIEMENS & EALSKE AKTIENGESELLSGHAFT, OF SIEMENSSTADT, NEAR BERLIN, GERMANY, A

CORPORATION OF GERMANY ELEGTBDLY'IIIC PRODUCTION OF TIN FROM ALKAII STANNATE OR ALKALI' STANNITE IiYES 1T0 Drawing. application flledjbtay 13, 1929, .Serial No. 362,883, and in Germany May 16, 1928.

My invention relates to improvements in the production of tin, and more specifically to the. electrolytic production of tin from alkali stannate or alkali stannite lyes.

5 The hitherto known and actually employed process for the electrolytic production of tin from alkali stannate or alkali stannite solutions-in conjunction with soluble or insoluble anodes has the'drawback that the tin de-' posited on the cathode is spongy and very porous, so that it is necessary to press it down continuously by rollers or the like in order to prevent it from dropping off the cathode. Only if very low current densities of the 5. order of about 50 amps. per-square metre are employed is a fairly dense and smooth deposit possible, but even thensuch deposit can be obtained only in thin layers.

. A more recent suggestion is to employ an electrolyte, which quantitatively contains only alkali stannate and besides this an addition of a colloid. This procedure leads to a satisfactory deposition of tin. but it is. then impossible to work with soluble anodes, be-

cause the tin then dissolves in the two as Well as in the four valence condition and oxidation of the lower valence 'tin to the higher valence tin is in this case practically impossible. 1 After extensive and thorough ,research I .3 have ascertained that the spongy condition of the deposit of tin in the known process may be accounted for by the presence of an excess of free alkali. For example there is described to obtain the tin in the form of thin, firmly adhering coats of high lustre and great density (tinned sheet iron) as well as in thick, solid plates of dull silvery appearance without any spongy formations. My improved process employs soluble anodes. Such an electrolyte works absolutely satisfactorily at all concentrations of tin from 0.05 p. 0. up-

wards to about 7 p. c. and at current densities from' 50 amps. per square metre up to 1600 amps. per square metre. Preferably the content of free alkali is reduced as far as ossible. When the concentration of free a ali rises considerably over 1.5 p. c. a dull, dirtygray tin deposit which is inclined to become spongy, is produced.

The process described above permits of a good solution of the problem for tin lyes of a concentration up to 7 p. c. If tin lyes of a higher concentration are used, it is found that it is generally impossible toavoid and increase in the content of free alkali beyond 1.5 p. 0. due to the occurrence of electrolysis.-

In a further development of the invention, especially for tin lyes of higher concentration, the deposition of spongy tin is avoided by adjusting the ratio of the content of tin to the content of free alkali at 5: 1. 5 'If, for instance, a tin 'lye is to be dealt with which contains 10 p, c. tin, the content of free alkali in this lye must be less. y

. As extensive experiments have shown, the adjustment of the ratio mentioned may be attained in all cases occurring in practice. I have also found that if the stated adjustment of the alkali content in relation to the adjusted to 2 p. c. or

content of-tin is made, the deposition of spongy tin'is also prevented in lyes of a concentration below? p. c. of tin, but it is possi-.

ble in such caseto observe the relative concentrations described hereinbefore. Iclaim:'--'

f 1. Aprocess for the electro-deposition of tin fromalkali stannate or alkali stannite lyes,'which comprises adjusting prior to the electrolysis the content of free alkali, including that liberated by hydrolysis, in depend ence on the tin content of the lye, so that the ratio between the free alkali and the tin content is approximately 1:5, said process is maintained during the electrolysis.

2.. A process for the electro-deposition of tin from alkali stannate or alkali stannite lyes having a. tin content below 7 comprising employing soluble anodes, whereby said ratio solu adjusting prior to the electrolysis the content of free alkali, including that liberated by hydrol sis, below 1.5%, said process employing le anodes, whereby said content of free alkali is maintained below 1.5% during electrolysis.

In testimony whereof I afiix my signature.

WERNER FAN SELAU. 

